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Setting safety Gaps = was Re: MURPHY ! AGAIN ??

At 02:17 AM 5/29/99 -0400, Sebrf-at-aol-dot-com wrote:
snip........ 

>
>		I do have a question for Terry about the proper setting for 
>the safety on the spark gap.  I usually set the safety gap about two gaps 
>behind my shorting bar.  The closer I get to what appears to be resonance, 
>the more activity I see between the shorting bar and the safety.  Should I 
>move the safety back further, or keep doing what I have been doing?


Hi Rich,

Thank's for defining "CATSCAN".  I got a few questions about it too... :-)

The safety gap settings this is pretty standard (I think):

	Connect the safety gap directly across your transformer's output with
NOTHING else connected in the circuit (no chokes, caps, resistors).  Then
input the rated voltage into the transformer.  In this state, your
transformer will simply put out its normal rated voltage.  Then you simply
find the point were the gap will just barely not fire.  Thus, you have
found the voltage that is just above the transformer's normal rating.  This
is were you want the safety gap to fire.  In a gap like mine, just make a
mark at that point and don't set the ground bar any further.  You could
simply always leave it at this point or keep it a few gaps ahead of the
main shorting bar if you want to be extra safe.  Just don't go beyond the
safe point your have marked.  Any safety gap can be set this way to insure
it will not arc at the rated voltage, but will soon after.  A variac that
goes up to 140 VAC is really nice for this setting.  If the safety gap does
not fire at 120 VAC input but fires at say 135 VAC input then you know
right were it is set at.  The setting should be checked periodically, as
the gap ages and time passes, to be sure it hasn't changed significantly. 

	With safety gaps that use bolts and such where the setting is rather
"tricky", I would measure the gap with feeler gauges (auto parts store) and
us them to set the gap.  The gauge could hold the precise distance while
one tightens the nuts which is usually pretty hard to do without messing up
the gap distance.

	If you are seeing too much activity of the safety gap, there may be
something wrong somewhere.  Perhaps the transformer is not putting out
equal voltages on each side.  This would be indicated by one side arcing
much more than the other.  You could use the adjustable gap in the above
setting test to determine this.  the high electrostatic fields around an
operating coil my affect the gap firing voltages too.  Perhaps moving the
gap further from the secondary action would help.  You may try placing a
quick aluminum foil grounded shield over the gap to see if that helps
excessive arcing.

As always, be very careful when doing this close work with the high voltage!

Cheers,

	Terry


>
>Rich Sebzda, With Adam and Kevin in the caboose.
>Your friendly neighborhood repairman of Computerised Axial Tomographic x-ray 
>Scanners. (I also do MRI but don't tell anyone)
>